The arrest warrant for Julian Assange should not stand and breaches "a matter of fundamental legal principle", the supreme court has heard .

Dinah Rose QC, defending the WikiLeaks founder in his final appeal against extradition to Sweden to face allegations of sex crimes, told the panel of seven senior judges that to consider the Swedish public prosecutor as a judicial authority was "contrary to a basic, fundamental principle of law".

Reaching back as far into European legal history as the Codex Iustinianus, dated 376AD, Rose said the Swedish prosecutor was a party in the Assange case and therefore not independent and impartial, breaching the principle that "no one should be judge in their own cause", which Rose said was one of the pillars of natural justice.

Opening the case for the Swedish judicial authority, Clare Montgomery QC said the arrest warrant was valid because judicial authorities, at least in the preliminary stages of investigations where arrest is being sought, need not be independent and impartial. She said police officers made decisions to arrest people and were not considered to be independent of the prosecuting authorities. Judicial, she argued, meant simply associated with the judicial process. "The decision whether to arrest somebody might be made by somebody who is partisan," she said. "That happens throughout Europe."

She was repeatedly questioned by the judges about the reasoning behind the assertion. She ended her opening remarks by referring to historic French definitions of judicial authorities which referred to both courts and prosecutors. She will resume her case today.

Assange was earlier greeted outside the UK's highest court by supporters playing protest songs and holding placards demanding his release from bail conditions.

Inside the packed court, Assange listened and took notes as Rose argued that the formation of a European framework decision on crossborder extradition arrangements showed "decisions with serious implications for personal liberties should only be taken by independent judicial authorities". She said: "The words 'judicial authority' can only be understood as meaning an independent judge or a person executing equivalent power." She said to include public prosecutors in the concept of what is a judicial authority was "contrary to a basic, fundamental principle of law".

The Europe-wide agreement on the application of extradition arrangements and subsequent concern about the definition of a judicial authority in the British parliament meant the high court verdict – that the arrest warrant was valid – was wrong, she added.

Assange left court through a scrum of photographers to the sound of more protest songs from supporters.

"I submit that when one looks not only at the context within which the framework decision was made, but the way it evolved from 1999 to 2002 and 2003 with the UK legislation there is a very strong indication that the intention was that, as a safeguard, the warrant should be issued by a judicial authority acting independently and executing a judicial function," she said in summation of a two-and-a-half-hour session of legal argument.

"We also submit it is clear that issue was a matter of considerable concern to parliament and also one can infer was matter of concern to the Council of Ministers, which explains the rejection of the commission's original proposal … there was great concern the proposal was too broad."

About 60 members of the public, media and supporters of Assange filled the public benches in the supreme court for the start of the two-day hearing which, so far, has not touched on any of the details of the allegations against the 40-year-old Australian in Sweden.

The case continues and is expected to conclude on Thursday afternoon when the judges are expected to reserve judgment.